1. Field of the Invention
The present invention relates to analog/digital information processing, and more particularly to analog-to-digital converting techniques by use of analog memories.
Although the present invention is principally utilized in A/D converters and particularly described herein in connection therewith, it should be understood that the present invention may be widely applied to a signal processor and so on.
2. Description of the Related Art
While analog memory devices represented by a charge coupled device (CCD) and other charge transfer devices (CTD) have been widely spread in the form of imaging devices and delay lines, they are merely applied to limited fields of their original uses such as analog shift registers, analog memories and so on. Although the utilization of these analog memory devices in matched filters and multi-value logic circuits have been investigated, very few applications have been actually realized in the field of such signal processing.
In general, the CCD has excellent characteristics such as less power consumption and high density of integration, so that the establishment of higher signal processing functions such as A/D conversion has been expected. However, no application has been so far reported for A/D conversion.
Although not included in the charge transfer device, there has been proposed and commercialized a method of realizing D/A and A/D conversion based on the charge scaling principle by applying "a switched capacitor circuit" which utilizes capacitance for an analog memory to perform signal processing (FIG. 1). However, with the switched capacitor circuit, it is indispensable to repetitively recharge and discharge capacitors in the circuit using a stable voltage source, so that required power consumption is much larger than that of the charge transfer device such as CCD. For this reason, it is generally thought that the implementation of the switched capacitor circuit in a high density integrated circuit and the enhancement of the operating speed are limited. The switched capacitor circuit further implies a number of disadvantages. For example, it cannot execute A/D conversion with a charge signal used directly as an input signal. For further details about the switched capacitor circuit, refer to an article entitled "VSLI design techniques for analog and digital circuits", by R. L. Geiger et al, McGRAW-HILL (Electronic Engineering Series), pp. 612-667 (1990).